Several processes have been disclosed for adding a feed material to a molten metal to form an alloy therewith, or to form an intermetallic compound. Such processes are referred to as "spark cup processes" because they convert the feed, e.g., lead, bismuth, tin, titanium, nickel, or other metal, into a superheated spray within a chamber or spark cup which is at least partially immersed in the molten metal.
The spark cup has a lower open end which is exposed to the molten metal and an upper inlet, which is located above the exposed or exterior surface of the molten metal. The lower open end of the spark cup is immersed to a predetermined depth below the surface of the molten metal. A wire is continually fed into the spark cup through its upper inlet. An electrical arc discharge, between the submerged molten metal surface and the end of the wire, is maintained with a current that exceeds the globular/spray transition current density of the feed wire. At such current, the free end of the feed wire is converted into a spray of superheated material which contacts and is alloyed into the molten metal.
Along with the feed wire, an ionizable gas is also continually supplied to the spark cup through its upper inlet. In addition to shielding the arc discharge, the gas slightly pressurizes the spark cup and prevents molten metal from entering its open end. As a result, the surface of molten metal within the spark cup is depressed relative to the surface of the molten metal outside the cup. The shielding gas also carries or projects the superheated spray of feed into the molten metal through the depressed surface so as to permit dissolution and dispersion of feed wire material into the molten metal. Depressing the melt surface within the cup relative to the surface of the molten metal outside the cup has been found to enhance significantly the dispersion and dissolution of the feed material into the molten metal. This prior art "spark cup" process is preferably used to make alloys of any molten metal, particularly of aluminum, and also to make intermetallic compounds such as TiAl.sub.3 and NiAl.sub.3. Further details of several spark cup processes are described in U.S. Pat. Nos. 4,688,771; 4,689,199; 4,784,832; 4,792,431 and 4,793,971, all issued to Eckert et al, the disclosures of which are incorporated by reference thereto as if fully set forth herein.
During the operation of the spark cup processes, the electric arc is so violent that molten metal frequently splashes up against its inner walls where it cools and solidifies. In a commercial operation, the coating of metal in the interior of the cup builds up to a point where it reduces the cross sectional area of the interior of the cup. The reduction of the interior of the cup makes the surface of the melt in the cup fluctuate out of control.
The prior art processes, therefore, focused on maintaining constant current with the expectation that the cup would provide a melt surface no more violent than a rippled one so that the complementary fluctuations in voltage would be tolerable. To the extent that the melt surface did not produce waves, as is the case when the feed rate is low and the amperage is correspondingly low, the spark process is acceptable because the cup life is not unexpectedly foreshortened.
The prior art spark cup process is not commercially acceptable at high feed rates and high current amperage because the spark cup is damaged with no forewarning, and the process must be stopped to replace the spark cup. The downtime associated with such interruptions are unacceptable in a production facility.
Moreover, the heat generated by the electric arc within the cup is such that, despite being made of boron nitride or other heat-tolerant ceramic material, the spark cup is damaged after a very short period of time. If the spark cup develops even a crack, it is unusable; the alloying process must be stopped until a replacement cup can be installed.
The unavoidable concomitant of the spark cup process is that the current requirement which is a function of feed rate, and the corresponding amount of heat generated (the I.sup.2 R effect) is not only very large but also highly variable even over short intervals of time from about 0.1 second to 1 second.
In addition, there are pressure fluctuations within the spark cup. The fluctuations are the result of gas building up until a sufficiently high pressure is reached to cause a sudden escape of gas from under the cup. The gas escapes as a bubble and normally flows to the surface of the melt. The sudden release of gas is followed by another gradual pressure build-up.
Such fluctuations in gas pressure cause dynamic oscillations of the depressed melt surface within the cup. Such oscillations of the depressed melt surface, in turn, lead to dynamic oscillations of current transmitted. These dynamic oscillations result in such instability as to make the spark cup process very difficult to operate continuously. Though the apparent solution to the problem lay in stabilizing the process, it was not evident how such stabilization may be effected.
The present disclosure is a description of how this is accomplished.